Digital high definition video data may be successfully transmitted over terrestrial television channels by generating compressed video data, partitioning the video data between high and low priority information, and quadrature amplitude modulating the high and low priority data on separate carriers respectively. The modulated carriers are included in a 6 MHZ frequency spectrum and then the combined signal is translated to occupy a standard broadcast channel spectrum. The high priority data is transmitted with relatively high power and the low priority data with relatively low power. High priority data is that video data which is sufficient to reproduce an image, albeit of lesser quality than a high definition image.
The present invention is directed to circuitry for separating compressed video data between relatively high and low priority video data. For purposes of this disclosure the video data will be presumed to be compressed in MPEG like format (though any data format which may be hierarchically layered is susceptible of use). What is meant by "MPEG like" is a coding format similar to the standardized coding format being established by the International Organization of Standardization. The standard is described in the document "International Organization for Standardization", ISO-IEC JT(1/SC2/WG1), Coding of Moving Pictures and Associated Audio, MPEG 90/176 Rev. 2, Dec. 18, 1990, which document is incorporated herein by reference for description of the general code format.
The MPEG standard transmits 240 lines (NTSC) per frame non-interlaced, which is typically accomplished by encoding only the odd or even fields of an interlaced source video signal. For transmitting HDTV signals the standard is modified to provide, for example 480 lines per field, and both the odd and even fields are transmitted. In addition the number of pixels per line is increased to for example 1440. Conceptually these changes only affect the data rate and do not affect the compression philosophy. Of particular interest regarding this coding format is that successive frames are encoded according to a cyclical sequence wherein particular frames of the sequence are intraframe encoded (I frames), other frames (P frames) are forward interframe encoded, and still other frames (B frames) are both forward and backward interframe encoded. The encoded signal format for frames of each of the encoding types is similar but the relative importance of the types of encoded frames for image reproduction is I, P, B. Images may be reproduced from single I frames, however image reproduction for P and B frames requires information derived from prior decoded I or P frames.
The amount of data bits for respective encoded frames varies widely. In addition the percentage of information nominally considered to be low priority data in respective frames may vary widely. As such allocating data between high and low priority channels is not a simple matter of simply parsing a particular K percent of the data for each frame to the high priority channel and the remaining (100-K) percent to the low priority channel. The parsing becomes further complicated if the relative importance of the encoded frame types is included in the prioritizing process.